We investigated by electron microscopy the ultrastructural character of the mature embryo sac in the chromosome-9 monosomic addition line of Beta corolliflora Zoss. in sugar beet, designated M14. The mature embryo sac of M14 consisted of an egg cell, two synergids, a central cell with a secondary nucleus, and 3-6 antipodal cells. The egg cells in different embryo sacs showed different polarity and metabolic activity. They were classified into three types according to the polarity of egg cells: (1) egg cell with regular polarity—the nucleus was located at the chalazal end, and its cytoplasm was rich in organelles such as ribosomes,
mitochondria and endoplasmic reticulum; (2) egg cell with a nucleus at the micropylar end, and (3) egg cell with a centrally located nucleus. The eggs of the third type contained fewer organelles in cytoplasm. The two synergids degenerated before anthesis in most mature embryo sacs. The secondary nucleus, which was close to the antipodals, showed strong metabolic activity. Both the egg cell and the central cell contained large nuclei with distinct nucleoli and numerous organelles. The egg and the central cell were suggested to be active in metabolism, which could be a label of the autonomous division of unfertilized egg cell and central cell in diplospory.